package com.liu.generator.impl;

import com.liu.generator.Generator;

public enum Generators implements Generator {

        IdWorker {
            /** 开始时间截 (2015-01-01) */
            private final long twepoch = 1420041600000L;

            /** 机器id所占的位数 */
            private final long workerIdBits = 5L;

            /** 数据标识id所占的位数 */
            private final long datacenterIdBits = 5L;


            /** 序列在id中占的位数 */
            private final long sequenceBits = 12L;

            /** 机器ID向左移12位 */
            private final long workerIdShift = sequenceBits;

            /** 数据标识id向左移17位(12+5) */
            private final long datacenterIdShift = sequenceBits + workerIdBits;

            /** 时间截向左移22位(5+5+12) */
            private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;

            /** 生成序列的掩码，这里为4095 (0b111111111111=0xfff=4095) */
            private final long sequenceMask = -1L ^ (-1L << sequenceBits);

            /** 毫秒内序列(0~4095) */
            private long sequence = 0L;

            /** 上次生成ID的时间截 */
            private long lastTimestamp = -1L;

            /**
             * 获得下一个ID (该方法是线程安全的)
             * @return SnowflakeId
             */
            public synchronized long nextId() {
                long timestamp = timeGen();

                // 如果当前时间小于上一次ID生成的时间戳，说明系统时钟回退过这个时候应当抛出异常
                if (timestamp < lastTimestamp) {
                    throw new RuntimeException(
                            String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
                }

                // 如果是同一时间生成的，则进行毫秒内序列
                if (lastTimestamp == timestamp) {
                    sequence = (sequence + 1) & sequenceMask;
                    // 毫秒内序列溢出
                    if (sequence == 0) {
                        // 阻塞到下一个毫秒,获得新的时间戳
                        timestamp = tilNextMillis(lastTimestamp);
                    }
                }
                // 时间戳改变，毫秒内序列重置
                else {
                    sequence = 0L;
                }

                // 上次生成ID的时间截
                lastTimestamp = timestamp;

                // 移位并通过或运算拼到一起组成64位的ID
                return ((timestamp - twepoch) << timestampLeftShift) //
                        | (0 << datacenterIdShift) //
                        | (0 << workerIdShift) //
                        | sequence;
            }

            /**
             * 阻塞到下一个毫秒，直到获得新的时间戳
             * @param lastTimestamp 上次生成ID的时间截
             * @return 当前时间戳
             */
            protected long tilNextMillis(long lastTimestamp) {
                long timestamp = timeGen();
                while (timestamp <= lastTimestamp) {
                    timestamp = timeGen();
                }
                return timestamp;
            }

            /**
             * 返回以毫秒为单位的当前时间
             * @return 当前时间(毫秒)
             */
            protected long timeGen() {
                return System.currentTimeMillis();
            }

            @Override
            public Long generateMessageID() {
                return nextId();
            }
        }
    }